As it is known, in the case of tin-plate the pre-plating technology has up to now developed along two lines. One line brought to the pre-plating method the use of a bath of an acid nature wherein the plating material is present in the cationic form (e.g. Sn.sup.+2, stannous ion).
The second line led to the use of an alkaline solution wherein the pre-plating metal is present in the anionic form (e.g. SnO.sub.3.sup.-2, stannate ion).
As it is known, to get a good plating it is necessary that the surface of the material to be plated is completely clean.
The prior art, as far as steel platings are concerned, requires that the article to be plated is previously sujected to suitable pre-treatment operations, i.e. subjected to operations such as surface preparation, cleaning and acid pickling.
However, this pre-treatment technology has several inconveniences in its practical use. Among these inconveniences there are the chemical agressiveness of the detergent and deoxidizing agents causing the decrease of plant life, and the problem of all the matters concerning maintenance, work hygiene and work safety.
Further, the discharge of the exhaust waters from the treatments contributes remarkably to the environmental pollution and the alteration of the local ecological and geochemical equilibria.
Finally, the depuration processes of the exhausted baths are considerably complicated from both the technical and economical point of view.
The above inconveniences are partially overcome by using as the pre-treatment a neutral electrolyte of sodium sulphate as described in U.S. patent application Ser. No. 726,788 now U.S. Pat. No. 4,127,450. Such a treatment consists essentially in the steel to be plated being made alternatively cathode and anode in an electrolysis cell. From the electrochemical point of view, said pre-treatment may be included in the following reaction scheme: EQU anode (positive pole) 2OH.sup.- .revreaction.H.sub.2 O+1/2O.sub.2 +2e EQU cathode (negative pole) 2H.sup.+ re .revreaction.H.sub.2 EQU global reaction 2OH.sup.- +2H.sup.+ .revreaction.H.sub.2 O+1/2O.sub.2 +H.sub.2
The reaction scheme shown clearly means that the cations and anions of the sodium sulphate and the other salts possibly present, do not participate in the redox semi-reactions cathodic and anodic respectively.
During the surface treatment in a neutral solution according to the above pre-treatment, there is consequently no consumption of solute, but only of OH.sup.- and H.sup.+ ions coming from the ionic dissociation of the water (whose amount therefore needs to be kept constant). Obviously, the activating action of the pre-treatment with respect to the surface depends on secondary reactions its components participate in.